Create new DesktopNavbarButtonsViewController class.

This CL creates a new DesktopNavbarButtonsViewController class to allow
for taskbar buttons and status icons customization on desktop
environment.

Recall: http://recall/clips/1f3b88e2-9f4e-4291-a9fe-a17641d1fd0c

Bug: 213964564
Bug: 196757951
Test: Switch between different system navigation modes and verify the
desktop taskbar buttons/status icons stay on screen, and are functional.

Change-Id: I05846b61689213ded88b8f49460aced88e49fd65

packages/SystemUI/shared/src/com/android/systemui/shared/rotation/RotationButtonController.java

@@ -16,6 +16,7 @@
 
 package com.android.systemui.shared.rotation;
 
+import static android.content.pm.PackageManager.FEATURE_PC;
 import static android.view.Display.DEFAULT_DISPLAY;
 
 import static com.android.internal.view.RotationPolicy.NATURAL_ROTATION;
@@ -200,7 +201,7 @@ public class RotationButtonController {
     }
 
     public void registerListeners() {
-        if (mListenersRegistered) {
+        if (mListenersRegistered || getContext().getPackageManager().hasSystemFeature(FEATURE_PC)) {
             return;
         }
 
@@ -414,6 +415,9 @@ public class RotationButtonController {
     }
 
     public void onTaskbarStateChange(boolean visible, boolean stashed) {
+        if (getRotationButton() == null) {
+            return;
+        }
         getRotationButton().onTaskbarStateChanged(visible, stashed);
     }